The invention relates to a wave form generator for sound formation in an electronic musical instrument with a storage unit having at least two portions, each of which has a set of storage cells for storing one wave form. The scanning values associated with a wave form at predetermined scanning points are stored in the storage cells. Each different storage cell is associated with a corresponding scanning point on a wave form. The generator also has a controllable store read-out device and a smoothing unit connected upstream of the wave form generator output.
Such a wave form generator is known from W. German Disclosure Paper DOS 2,830,483 (K.K. Suwa Seikosha, application date July 11, 1978). In one of the two wave form stores of this known wave form generator, the scanning values of the wave form for a solo melody are stored, while in the other wave form store the scanning values of the wave form of accompanying music are stored. An envelope circuit is connected downstream of each function store for further influencing the sound quality. The smoothing unit connected upstream of the wave form generator output has in particular a digital-to-analog converter or a low-pass filter. The smoothing unit is used for reducing the harmonic content of a stepped wave form, leading to a better simulation of natural tones or notes. In this known device, there is no possibility of modifying the time dependence on the curve shape set by means of the wave form stores and/or the envelope circuit.
W. German Publication Paper DAS 2,237,594 (Nippon Gakki Seizo K.K.; application date July 31, 1972) discloses a wave form generator for an electronic musical instrument having a wave form store which can be read out and also having a set of resistance elements. The scanning values of a wave form are stored in the function store and the resistance values of the resistance elements are set in such a way that, in analog form, their scanning values represent the amplitudes of the wave form. As a result of this type of store design, the disadvantages of digital stores as known from e.g. W. German Disclosure Paper 1,935,306 (Allen Organ Corporation, application date April 15, 1976) are eliminated. However, in this known device, there is no possibility of cross-fading from one wave form to another for changing the spectral synthesis of a note.
It is also known that wave forms with random configurations can be produced with the aid of shift registers, e.g. in the form of ring counters (one of N counters). In this connection, a location in the shift register is associated with the voltage value at a given time, the voltage value generally being in the form of a parallel binary word. Thus, this shift register is able to process a plurality of bits in parallel. In this case, the output is a random location between two shift register cells at which successively appear all the (binary coded) voltage values which occur, during a cycle i.e. which are written into the shift register cells. Advantageously, the Nth, i.e. the last shift register cell is selected as the output. At the end of the cycle, the process starts anew, because the Nth output is fed back to the first input. Thus, the voltage value information stored once in each storage cell travels round continuously. The cycle time is obtained from the product: clock frequency reciprocal x number of storage cells, i.e. T=.DELTA.T.times.N, with .DELTA.T=1/F (clock); N=number of storage cells.
From the German Journal "elrad" 1979, No. 5, page 28, title "Harmonization of digitalised curves", it is known to use a so-called "deglitcher" as the smoothing or interpolation unit for harmonizing digitalised wave forms. The known deglitcher has two sample/scanning and hold elements operating at a common switching point that feeds an input of an integrator and the integrator output is fed back to the input of one sample/scanning and hold element. The feedback takes place in such a way that the difference between the signal values at the integrator output and the sample/scanning and hold elements respectively appear at the common switching point.